1. Field of the Invention
The present invention relates to a coating apparatus coating a coating liquid on a web surface.
2. Description of the Related Art
A conventional coating apparatus will be outlined below based on FIGS. 9 and 10. FIG. 9 is a schematic side view of a prior art example of a coating apparatus, as disclosed by the Japanese laid-open patent application 2002-263549 as Patent Document 1 (pages 4 and 6, FIGS. 1 and 5). The coating apparatus of FIG. 9 is constructed such that two applicator rolls are provided abutting on each other so as to form a nip portion therebetween and films of a coating liquid applied to surfaces of the two applicator rolls are transferred onto both sides of a web passing through the nip portion. FIG. 10 is a schematic side view of another prior art example of a coating apparatus.
In FIG. 9, a lower applicator roll 2a and an upper applicator roll 2b are opposedly provided so as to abut on each other and so as to form a nip portion 4 therebetween. A web 1 of paper or the like transferred from a previous process passes through the nip portion 4 to thereby be coated with a coating liquid and then, via a turn bar (an air levitating type non-contact turn bar) 6 that transfers the web 1 while the web 1 is being levitated by air, enters a drying device 40. The applicator rolls 2a, 2b comprise roll bodies 21a, 21b, respectively, made of a metal such as steel or the like, and elastic sheets 22a, 22b, respectively, of rubber or the like applied to outer circumferential surfaces of the roll bodies 21a, 21b, and rotate with a circumferential velocity the same as a running velocity of the web 1. On an upstream side of the nip portion 4 in respective rotational directions of the applicator rolls 2a, 2b, a lower coater head 3a and an upper coater head 3b, respectively, are provided as devices supplying the coating liquid onto outer surfaces of the respective elastic sheets 22a, 22b. 
The respective coater heads 3a, 3b as coating liquid supply devices comprise metering devices including coating liquid supply pipings, metering rods, blades, etc. (not shown). When the coating liquid is sufficiently supplied onto the surfaces of the applicator rolls 2a, 2b from the coater heads 3a, 3b, the metering rods provided at outlets of the coater heads 3a, 3b are pressed against the surfaces of the applicator rolls 2a, 2b so that coating liquid films of a predetermined film thickness are formed on the surfaces of the applicator rolls 2a, 2b. 
While the web 1 passes through the nip portion 4 where the applicator rolls 2a, 2b abut on each other, the coating liquid films formed on the surfaces of the applicator rolls 2a, 2b make contact with surfaces of respective sides of the web 1 and are transferred to be coated thereon. While the web 1 coated with the coating liquid is being transferred toward the drying device 40 via the run bar 6, the turn bar 6 supports the web 1 to be levitated from the surface of the device by the force of air so that the surface of the web 1 may not make contact with a supporting member. Thus, the web 1 can be transferred into the drying device 40 without damaging a quality of the coated surfaces formed on the web 1 which are not yet dry.
On the other hand, when the coating liquid is transferred to be coated on the web 1, if the web 1 is of a water absorptive nature, such as paper or the like, elongation or contraction of the web 1 is caused by the absorption of water.
If the web 1 is arranged to linearly pass through the nip portion 4 in a tangential direction of both of the applicator rolls 2a, 2b, as shown by a dotted line in FIG. 9, as no such a device or member as to restrict a passing route of the web 1 that has passed through the nip portion 4 is provided in the way to the turn bar 6, a problem may be caused in that the web 1 that has elongated on a downstream side of the nip portion 4 runs sticking to the surface of any one of the lower and upper applicator rolls 2a, 2b by an adhesive action of the coating liquid. If the web 1 has a large width in an axial direction of the applicator rolls 2a, 2b, this sticking state becomes irregular in a width direction of the web 1 and a vibrating state may be caused. Especially, if the coating is to be done at a high speed, this irregular state begins to vary as time passes and this may lead to a very unstable situation.
As the result thereof, a coating irregularity called “peeled pattern” may be caused and also a “misting” phenomenon may arise in which the coating liquid becomes mist and scatters. If this phenomenon arises, the coating apparatus and the coated paper are contaminated and this may lead to an operation obstacle. Also, as a contact distance with which the web 1 makes contact with the applicator rolls 2a, 2b is as short as the nip portion 4 only (about 20 mm), contact and penetration of the coating liquid with and into the web 1 are often insufficient and this may invite a deterioration of the coating state and a generation of the misting phenomenon.
Hence, in the coating apparatus shown in FIG. 9, a roll side turn bar 5 of an air levitating type is provided on the downstream side of the nip portion 4 so that the web 1 immediately after passing through the nip portion 4 is kept supported on the surface of the upper applicator roll 2b and transferred. Also, a paper roll 7 is provided on the upstream side of the nip portion 4 in the running direction of the web 1 so that the web 1 immediately before entering the nip portion 4 is kept supported on the surface of the lower applicator roll 2a and transferred. At the roll side turn bar 5, like in the case of the turn bar 6, the web 1 is supported by being levitated from the surface of the device by the force of air so that the surface of the web 1 may not make contact with a supporting member and thus, the web 1 can be transferred without damaging the quality of the coated surfaces on the web 1 which are not yet dry.
That is, by the arrangement of the roll side turn bar 5 of an air levitating type, the web 1 immediately after passing through the nip portion 4 is kept supported on the surface of the upper applicator roll 2b and transferred to then be peeled off from the upper applicator roll 2b with a large peeling angle β formed between the web 1 and the surface of the upper applicator roll 2b. Thereby, the peeling action and the peeling position are stabilized and the coating irregularity, such as the peeled pattern for example, and the misting phenomenon can be prevented. Also, the contact distance of the web 1 in contact with the upper applicator roll 2b is elongated than in the case of the nip portion 4 only. Thereby, the penetration of the coating liquid into the web 1 is accelerated and the coating state is improved.
Also, by the arrangement of the paper roll 7, the web 1 immediately before entering the nip portion 4 is transferred being kept supported on the surface of the lower applicator roll 2a. Thereby, the contact distance of the web 1 in contact with the lower applicator roll 2a is elongated than in the case of the nip portion 4 only. Hence, the penetration of the coating liquid into the web 1 from the lower applicator roll 2a is accelerated, the coating state is improved and the misting phenomenon on the downstream side of the nip portion 4 is reduced.
Moreover, the contact distance of the web 1 in contact with the lower applicator roll 2a on the upstream side of the nip portion 4 and the contact distance of the web 1 in contact with the upper applicator roll 2b on the downstream side of the nip portion 4 can be set substantially equally to each other, so that an effect is obtained in that the coating states of both sides of the web 1 can be easily equalized.
Nevertheless, recently, there are large demands for coating apparatuses of more and more high speed, such as a web velocity of 2000 m/min or more for example. Thus, even in the conventional coating apparatus improved as mentioned above, if it is operated at the high speed, an entrained air flow of the web 1 is caused as well as an entrained air flow of the lower applicator roll 2a on the upstream side of the nip portion 4, and these entrained air flows enter between the web 1 and the lower applicator roll 2a. Thereby, the web 1 is levitated on the upstream side of the nip portion 4 to be prevented from making contact with the coating liquid on the lower applicator roll 2a and there arises a phenomenon that when the web 1 reaches the nip portion 4, it first makes contact with the coating liquid. Thus, there is a problem to nullify the effect of the paper roll 7 to urge the web 1 to make contact with the surface of the lower applicator roll 2a on the upstream side of the nip portion 4. Also, by the web 1 so making less contact with the lower applicator roll 2a on the upstream side of the nip portion 4, the transfer of the coating liquid onto one side of the web 1 on the lower applicator roll 2a side, that is, the lower side of the web 1 in FIG. 9, becomes deteriorated and a problem arises in that an irregularity of the coating state is caused between each side of the web 1. It is to be noted that these problems will likewise arise in such an arrangement of the coating apparatus that the arrangement of the devices of FIG. 9 is turned upside down.
While the above-mentioned prior art example is a coating apparatus in which the two applicator rolls arranged to abut on each other form the nip portion 4 and the web 1 passes through the nip portion 4 so that the coating liquid films formed on the two applicator rolls are transferred to be coated on both sides of the web 1, there is also a similar problem in such a coating apparatus as disclosed by the Japanese laid-open patent application 1996-144196 as Patent Document 2 (page 4, FIG. 1) in which the nip portion is formed between an applicator roll and a backing roll and coating is carried out on one side of the web 1.
FIG. 10 shows an example of such a coating apparatus. There, a curtain coating method is carried out such that a curtain die 51 as a coating liquid supply device forms a curtain-like form of a coating liquid falling so as to be supplied onto a circumferential surface of a rotating applicator roll 52 so that a uniform coating liquid film is formed thereon and, at a nip portion 54 between the applicator roll 52 and a backing roll 53, the coating liquid film is transferred onto a web 1 running being wound around the backing roll 53. Then, a surplus coating liquid is scraped off by a doctor plate 56. The web 1 is urged to be wound around the backing roll 53 by a guide roll 55 provided on the upstream side of the nip portion 54 and runs into the nip portion 54 via a wind breaking plate 57. The coating liquid is supplied from a coating liquid tank 59 by a coating liquid pump 58 to be sent to the curtain die 51 and supplied onto the applicator roll 52, as mentioned above, on the upstream side of the nip portion 54.
In the arrangement of the coating apparatus as mentioned above, entrained air flow on the circumferential surface of the applicator roll 52 is liable to enter the nip portion 54 and also entrained air flow on the web 1, despite the function of the wind breaking plate 57, often enters the nip portion 54 and this easily gives a bad influence on the coated state of the web 1. Hence, even in the coating apparatus in which the coating is carried out on one side of the web only, it is necessary to remove the entrained air flow on the web as well as on the applicator roll.
However, at present, a sufficiently improved coating apparatus has not yet been provided that is able to effectively remove not only the entrained air flow of the web but also the entrained air flow of the applicator roll. For example, the Japanese laid-open patent application 2003-326210 as Patent Document 3 (page 5, FIG. 2), that is a related invention to the present invention, relates to an improvement of the roll side turn bar 5 of an air levitating type as an improvement of the apparatus of the above-mentioned Patent Document 1.